This is an issue I was experiencing as well. Enabling the GuC does nothing if it cannot load the firmware. Head to the site it references and download ehl_guc_62.0.0.bin. Place it in /lib/firmware/i915/ and reboot. If hardware accelerated transcoding still fails, report the same output from dmesg and server logs again so we can see if the firmware was found.
(Here’s a direct link to the file, if you want to use that.)
